Coiled tubing injector with a weight on bit circuit

ABSTRACT

A control circuit for a coiled tubing injector includes a circuit controller ( 14 ). One or more counterbalance valves ( 3 ) may be responsive to the circuit controller to prevent the tubing string from descending into the wellbore during rate of penetration movement. In one embodiment, a control valve ( 4 ) allows fluid to flow through a bypass line around the counterbalance valves when drilling in a weight-on-bit mode. Other hydraulic control circuits allow for the release of fluid pressure to control the injector hydraulic motor. In yet another embodiment, the control circuit regulates power to an electric motor of the injector.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the priority of U.S. Provisional Application No.60/846,721 filed on Sep. 22, 2006, the disclosure of which isincorporated herein by reference for all purposes.

FIELD OF THE INVENTION

The present invention relates to coiled tubing of injectors of a typecommonly used in oilfield operation to raise or lower coiled tubing intoa well. More particularly, the invention relates to a hydraulic circuitfor controlling the drive motors of a coiled tubing injector so as tomore reliably control weight on bit at various depths.

BACKGROUND OF THE INVENTION

Coiled tubing injectors have been used for years to inject coiled tubinginto a well, while a drawworks and/or a top drive are conventionallyused for raising and lowering threaded tubulars into a well. In eithercase, weight on bit circuits have been devised to control deploying thecoiled tubing string or the threaded tubular string into the wellboresuch that the rate of penetration is determined by the weight-on-bit(WOB) rather than a fixed rate of penetration.

A conventional top drive feed system typically uses hydraulic cylinderscoupled to the top drive (TD) with a cable, chains, rack and piniongears, or directly. The cylinders use hydraulic pressure to hold the topdrive from traveling downwards, thereby preventing the drill string fromdescending into the borehole. In this conventional system, the operatormay allow the top drive to descend by bleeding off hydraulic pressure tothe feed cylinders. This is accomplished by using an operator adjustablerelief valve. For example, if the feed system requires 2000 psi tosupport the drill string, the operator may set the relief valve to 1500psi. Hydraulic fluid will then bleed off from the cylinder to thehydraulic tank and the drill string descends into the well bore. Thispressure differential may result in an exemplary weight on bit of about20,000 lbs.

The hydraulic system will always maintain this pressure differential. Ifthe drill bit encounters a more difficult formation, the rate ofpenetration will slow. Conversely if the bit encounters a softerformation, the bit will descend more rapidly. At all times, the pressuredifferential will be 500 psi and the WOB will still be about 20,000 lbs.

The top drive (TD) travel may be governed by electrical motor, hydraulicmotor, or mechanical means using a drawworks winch. In these cases, thesystem is frequently called an “auto drill,” and consists of amechanical drawworks lowering the drill string into the well bore. Abrake on the drawworks controls decent and modulating the brakeautomatically accomplishes the same goal. More complex systems useelectric or hydraulic drive motors on the drawworks controlled withPLC's (programmable logic controls).

Once a sufficient amount of drill pipe and a bottom hole assembly (BHA)is in the hole at the lower end of the drill pipe, the top drive feedsystem ceases to push the dill string into the hole. Instead, thedesired to “weight on bit” may be obtained by restricting but notpreventing the drill string from descending into the hole, so that thedesired rate achieves the desired weight on bit.

Various techniques have been devised for controlling a drilling system,and particularly a coiled tubing drilling system which utilizes aninjector. U.S. Pat. No. 4,585,061 discloses a system for inserting andwithdrawing coiled tubing with respect to a well. Another technique forinjecting coiled tubing into a well is disclosed in U.S. Pat. No.5,839,514. U.S. Pat. No. 6,047,784 discloses the use of a sensor in acoiled tubing system to control weight-on-bit. U.S. Pat. No. 5,291,956also discloses controlling a coiled tubing injector assembly. U.S. Pat.No. 5,842,530 discloses a telescopic injector frame, and U.S. Pat. No.6,923,273 discloses different techniques for selectively obtaining adesired weight-on-bit for a tubing system. U.S. Pat. No. 6,536,539discloses the use of coiled tubing in a horizontal drilling system.

The disadvantages of the prior art are overcome by the presentinvention, and an improved circuit controller for controlling a coiledtubing injector is hereinafter disclosed.

SUMMARY OF THE INVENTION

In one embodiment, a control circuit for a coiled tubing injectorcontrols the raising or lowering of coiled tubing into a well. Theinjector includes an injector motor to power an injector drivemechanism, which engages the coiled tubing to raise or lower the coiledtubing. The circuit includes a circuit controller, and a counterbalancevalve opening in response to the circuit controller to power theinjector motor and closing in response to the circuit controller toprevent the tubing string from descending into the wellbore. A bypassline is provided around the counterbalance valve, and a control valvemay be opened to bypass the counterbalance valve. The circuit controllervaries power to the motor when the control valve is opened to raise orlower the coiled tubing.

In various embodiments, the control circuit varies power to the coiledtubing injector motor to maintain a substantially constantweight-on-bit. More particularly, the control circuit controls theinjector motor to exert a controlled upward force on the coiled tubingthrough the injector drive mechanism, although this upward force is lessthan the drill string weight, thereby resulting in a substantiallyconstant weight-on-bit. Other embodiments of a hydraulic control circuitare provided and an exemplary electrical control circuit is disclosed.

It is a feature of the present invention that the control circuit forcontrolling the coiled tubing injector may be operable in the “in-hole”mode which results in rate of penetration drilling, and also in theweight-on-bit mode, wherein the control circuit exerts a controlledopened force on the coiled tubing less than the coiled tubing stringweight, thereby resulting in a substantially constant weight-on-bit.

These and further features and advantages of the present invention willbecome apparent from the following detailed description, whereinreference is made to the figures in the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a suitable hydraulic circuit for controlling a coiled tubinginjector according to one embodiment of the invention.

FIG. 2 is an alternative hydraulic circuit for controlling a coiledtubing injector.

FIG. 3 is yet another embodiment of a hydraulic circuit for controllinga coiled tubing injector.

FIG. 4 is an electrical circuit for controlling a coiled tubinginjector.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In one embodiment, an injector feeds coiled tubing into the well boreusing a hydraulic motor to power the chains that carry the gripperblocks. At equilibrium, the hydraulic motor is holding the drillingstring with a given pressure, e.g., approximately 2000 psi. Thatpressure may be supplied by a variable displacement, variable pressurehydraulic pump. The pump will thus be supplying 2000 psi pressure to thehydraulic motor, but there would be no flow of hydraulic fluid. Themotor is stationary and holding the load by the supplied pressure.

The hydraulic circuit incorporates a counterbalance valve. The functionof this valve is to prevent the load (the drill string) from descendinginto the well bore unless the injector circuit controller is in the“in-hole” mode. Selecting the in-hole mode opens the counterbalancevalve, allowing the hydraulic motor to power the drill string into theborehole. The counterbalance valve also serves the function of a safetydevice. Should all hydraulic pressure be lost, the counterbalance valvecloses, preventing the drill string from descending into the well bore.The in-hole mode results in “rate of penetration drilling”, where thedrill string is being pushed into the well bore.

When the operator selects the “weight-on-bit” mode, the counterbalancevalve is bypassed. This is accomplished with an electrically operatedsolenoid valve that opens and provides an unrestricted path for thehydraulic fluid around the counterbalance valve. The drill string is nowsolely supported by the hydraulic pressure supplied by the pump to themotor, again at 2000 psi. The operator may then adjust the pressure ofthe pump to a lower value, e.g., 1500 psi, and the hydraulic motor willno longer be able to support the load and will run backwards, allowingthe drill string to descend into the well bore. Again this pressuredifferential results in a given WOB.

In a conventional or “rate of penetration” mode, the operatormanipulates the circuit controller 14 which controls the hydraulic pump1 as shown in FIG. 1 to cause the hydraulic pump 1 to provide hydraulicfluid flow to hydraulic motor 2 to provide rotational torque to theinjector drive mechanism 10. Two motors 2 are conventionally provided inparallel to power the gripper chains of the injector, although only onemotor is shown for simplicity in FIG. 1. The injector then feeds thecoiled tubing string into the ground. The operator is controlling therate of descent of the coiled tubing drilling string by varying the flowwith the circuit controller 14. In this mode, the counterbalance valve 3is opened by a pilot signal 11 from the pressure line 12 extending fromthe inlet side of the motor 2. A hydraulic pressure signal 13 from theinlet side of the motor 2 causes hydraulic valve 6 to open, allowinghydraulic pressure 16 to hold released a spring applied hydraulicallyreleased brake 9. Since the rate of decent is constant, the weight onbit increases as the bit encounters obstacles or harder formations. Twocounterbalance valves are shown in FIG. 1. When both valves are closed,the motor is stalled. One counterbalance valve thus acts to controllablylower the coiled tubing string in the hole, while the othercounterbalance valve is used when controllably raising the tubingstring.

When the operator manipulates a control 14 to a neutral position, thehydraulic pump 1 ceases to provide hydraulic fluid flow to the hydraulicmotor 2. The counterbalance valve 3 closes as there is no longer a pilotsignal 13 from the pressure line 12 to the inlet of the motor 2. Springapplied hydraulic released brake 9 is applied as pilot signal 13 isremoved and hydraulic valve 6 shifts, removing hydraulic pressure source16 from brake 9. The application of the brake 9 prevents the weight ofthe coiled tubing drill string from overpowering the hydraulic motor 2which would allow the string to descend into the hole unchecked. Thecounterbalance valve 3 acts as a back-up to the brake 9. Thecounterbalance valves hydraulically lock the motor 2 to prevent themotor from rotating backwards which would also allow the string todescend into the hole unchecked.

In certain drilling conditions, it is desirable for the operator tocontrol the weight on bit of the coiled tubing drill string rather thanthe rate of penetration. To accomplish this, the operator manipulatesthe controls 14 to withdraw the coiled tubing drill string from the wellbore by selecting a “pull out the hole” position on the controls. Theoperator may allow the weight of the coiled tubing string to descendinto the well bore by reducing the applied pressure to the hydraulicmotor by a pressure control valve 15. The weight of the coiled tubingdrill string then exceeds the ability of the hydraulic motor 2 to holdthe drilling string in position. Accordingly, the motor 2 reversesdirection and the string descends into the well bore. If the stringencounters an obstacle, the weight on bit would begin to increase andthe hydraulic pressure holding back the string would then begin todecrease. The pump 1 will sense the reduction of pressure in line 12,and cause the pump 1 and motor 2 to slow the rate of decent or evenreverse the direction of the pump 1 and motor 2 to maintain thehydraulic pressure the operator has set via the pressure control valve15. In this drilling mode, the brake 9 is prevented from applying byproviding hydraulic pressure signal from an external source line 16. Byswitching to weight on bit mode, the operator opens a hydraulic valve 7to allow this pressure source to release the brake. The counterbalancevalve 3 is bypassed in this mode to allow the motor to run in thereverse direction. By switching to the weight on bit mode, the operatoropens hydraulic valve 5 which allows an external hydraulic pressuresignal to open valve 4 which causes the hydraulic fluid to bypass thecounterbalance valve 3.

If the bit encounters a softer formation or a void, the hydraulicpressure at the inlet of motor 12 will increase as the weight of thestring causes the injector drive mechanism 10 to rotate the hydraulicmotor 2 in an “in hole” direction. Sensing this increase in pressure,the hydraulic pump 1 will attempt to reduce the pressure in line 12 tothe set point as established by hydraulic valve 15 by providing morehydraulic fluid flow to the hydraulic motor 2.

A significant improvement in reliably operating the injector is achievedby providing a complete bypass to the counterbalance valve. In practice,the counterbalance valve presents a restriction and when attempting toreverse fluid through the counterbalance valve, the system may notproperly operate at a desired weight-on-bit at various well depths. Byproviding a system which bypasses the counterbalance valve instead oftrying to pass fluid back through the counterbalance valve, hydraulicvalve 7 is open to allow pressure to release the brake and hydraulicvalve 5 is opened so that the external hydraulic pressure opens valve 4,which then causes hydraulic fluid to bypass the counterbalance valve.

As a modification to the above-described system, the hydraulic circuitfor controlling the injector may be responsive to a WOB signal from asensor, which may be a surface WOB sensor or may be a downhole sensor.In either case, the hydraulic circuit may use the weight-on-bit sensorrather than sensing a pressure change acting on the motor to determinethe varying weight on bit. A WOB sensor signal may then be input to acontrol valve which regulates the fluid flow rate applied to theinjector motors, and thereby controllably lower or raise the tubing withthe injector to obtain the desired weight on bit. If the actual WOBdecreases, the signal may operate the motors to lower more tubing intothe well to increase to actual WOB. If the actual WOB increases, thetubing may be raised (or lowered more slowly) until the actual WOBincreases.

FIG. 2 Circuit

An alternative hydraulic circuit will now be discussed, first in the“non-holdback” mode of operation and then in the holdback mode. In a“rate of penetration” drilling system using a coiled tubing injector,the operator may manipulate the hydraulic pump controls 34 to cause thehydraulic pump 31 to provide hydraulic fluid flow to a motor 32 toprovide rotational torque to the injector drive mechanism. The injectorthen feeds the coiled tubing string into the ground. The operator iscontrolling the rate of descent of the coiled tubing drilling string byvarying the flow with the pump controls 34. In this mode, thecounterbalance valve 33 may be opened by a pilot signal off the pressureline 42 from the inlet side of the motor 32. A spring appliedhydraulically released brake 39 may be held open by a hydraulic pressuresignal from the pressure line on the inlet side of the motor 32. Sincethe rate of decent is constant, the weight on bit increases as the bitencounters obstacles or harder formations.

When the operator manipulates a control 34 to a neutral position, thehydraulic pump 31 ceases to provide hydraulic fluid flow to thehydraulic motor 32. The counterbalance valve 33 closes as there is nolonger a pilot signal off the pressure line to the inlet of the motor32. A spring applied hydraulically released brake 39 may be applied asthe hydraulic pressure supplied by a hydraulic signal is removed. Theapplication of the brake 39 prevents the weight of the coiled tubingdrill string from overpowering the hydraulic motor 32, therebypreventing the string from descending into the hole unchecked. Thecounterbalance valve 33 thus acts as a back-up to the brake 39, andhydraulically locks the motor 32 to prevent it from rotating backwardswhich would also allow the string to descend into the hole unchecked.

In certain drilling conditions, it is desirable for the operator tocontrol the weight on bit of the coiled tubing drill string rather thanthe rate of penetration. To accomplish this, the operator may manipulatethe controls 34 to “hold back” the coiled tubing drill string byselecting an out hole position on the controls. The operator adjusts theout hole pressure using the pressure control valve 37 until the drillsting is suspended in the hole at equilibrium, i.e., the hydraulicpressure supporting the drilling string is sufficient to hold the weightof the drill string but not lift it upwards out of the hole. Then theoperator allows the weight of the coiled tubing drill string to descendinto the well bore by bleeding off the applied pressure on the loadsupporting side of the closed loop hydraulic drive system by a pressurerelief valve 38 to the inlet side of the closed loop circuit. The weightof the coiled tubing drill string then exceeds the reduced pressure inthe load supporting side of the closed loop drive, so that the drillstring descends in the hole. The motor 32 rotates in the in holedirection to allow the drill string to descend. As the motor rotates,replenishing hydraulic fluid in the closed loop system is supplied by ableed line 38. The drill string will descend as fast as the pressuredifferential between the load bearing side of the closed loop circuitand the inlet side will allow.

This alternative hydraulic system will strive to maintain this pressuredifferential at the operator set value. If the string encounters anobstacle, the weight on bit would begin to increase and the hydraulicpressure holding back the string would then begin to decrease. Tomaintain the pressure differential, the rate of descent would slow,thereby returning the pressure differential to the set value andconsequently maintaining the weight on bit at a constant value.Conversely, if the bit encounters a softer formation or a void, theapplied weight on bit would decrease. The pressure differential wouldthen decrease and the system would attempt to regain the pressuredifferential allowing the drill string to descend faster.

FIG. 3 Circuit

Yet another hydraulic system in the “non hold back” drilling mode mayfunction as follows. The operator may manipulate the hydraulic pump 31with controls 34 to cause the hydraulic pump 31 to provide hydraulicfluid flow to a hydraulic motor 52 to provide rotational torque to theinjector drive mechanism, as shown in FIG. 3. The injector then feedsthe coiled tubing string into the ground. The operator controls the rateof descent of the coiled tubing drilling string by varying the flow withthe pump controls 34. In this mode, the counter balance valve 33 isopened by a pilot signal off the pressure line 42 from the inlet side ofthe motor 52. A spring applied hydraulically released brake 39 is heldopen by a hydraulic pressure signal from the pressure line on the inletside of the motor 52. Since the rate of decent is constant the weight onbit increases as the bit encounters obstacles or harder formations.

When the operator manipulates a control 34 to a neutral position, thehydraulic pump 31 ceases to provide hydraulic fluid flow to thehydraulic motor 52. The counterbalance valve 33 closes as there is nolonger a pilot signal off the pressure line 42 to the inlet of the motor52. The spring applied hydraulically released brake 39 is activated asthe hydraulic pressure supplied by a pressure line 42 is removed. Theapplication of the brake 39 prevents the weight of the coiled tubingdrill string from overpowering the hydraulic motor 52 which would allowthe string to descend into the hole unchecked. The counterbalance valve33 acts as a back-up to the brake 39 and hydraulically locks the motor52 to prevent it from rotating backwards which would also allow thestring to descend into the hole unchecked.

In the hold back mode, this alternative hydraulic circuit functions asfollows. In certain drilling conditions, it is desirable for theoperator to control the weight on bit of the coiled tubing drill stringrather than the rate of penetration. To accomplish this the operatormanipulates the controls 34 to “hold back” the coiled tubing drillstring by selecting an out hole position on the controls. The operatorthe adjusts the out hole pressure using the pressure control valve 37until the drill sting is suspended in the hole at equilibrium. That isthe hydraulic pressure supporting the drilling string is sufficient tohold the weight of the drill string but not lift it upwards out of thehole. Then the operator allows the weight of the coiled tubing drillstring to descend into the well bore by bleeding off the appliedpressure on the load supporting side of the closed loop hydraulic drivesystem by a pressure relief valve 48 to the hydraulic reservoir line 50.The weight of the coiled tubing drill string then exceeds the reducedpressure in the load supporting side of the closed loop drive the drillstring descends in the hole. The motor 32 rotates in the in holedirection to allow the drill string to descend. As the motor rotates,replenishing hydraulic fluid in the closed loop system is supplied bythe hydraulic pump 31. The drill string will descend as fast as thepressure differential between the load bearing side of the closed loopcircuit and the inlet side will allow.

The system would always strive to maintain this pressure differential atthe operator set value. If the string encounters an obstacle the weighton bit would begin to increase and the hydraulic pressure holding backthe string would then begin to decrease. To maintain the pressuredifferential the rate of descent would slow thereby returning thepressure differential to the set value and consequently maintaining theweight on bit at a constant value as well. Conversely if the bit is toencounter a softer formation or a void the applied weight on bit woulddecrease. The pressure differential would then decrease and the systemwould attempt to regain the pressure differential allowing the drillstring to descend faster.

FIG. 4 Circuit

The technique of this invention may also function with an electricalcircuit to control power to an electric motor (5) of a coiled tubinginjector. In the “non-holdback” mode, the electrical circuit functionsas follows. Referring to FIG. 4, the operator manipulates the electricdrive 61 using control 62 to cause the electric drive to in holeamperage controlled at 63 and out hole amperage controlled at 62 to oneor more electric motors 62 for the injector to provide rotational torqueto the injector drive mechanism 64. The injector then feeds the coiledtubing string into the ground. The operator is controlling the rate ofdescent of the coiled tubing drilling string by varying the amperagewith the controls 62 and 63. Since the rate of decent is constant, theweight on bit increases as the bit encounters obstacles or harderformations.

In the hold back mode the electric circuit functions as follows. Incertain drilling conditions, it is desirable for the operator to controlthe weight on bit of the coiled tubing drill string rather than the rateof penetration. To accomplish this, the operator manipulates thecontrols to “hold back” the coiled tubing drill string by selecting anout hole position on the controls. The operator adjusts the out holeamperage using the amperage control until the drill sting is suspendedin the hole at equilibrium, i.e., the electric amperage supporting thedrilling string is sufficient to hold the weight of the drill string butnot lift it upwards out of the hole. Then the operator allows the weightof the coiled tubing drill string to descend into the well bore byreducing the amperage supporting the load using control 62. The motor 66rotates in the in hole direction to allow the drill string to descend.The system will strive to maintain this amperage at the operator setvalue. If the string encounters an obstacle, the weight on bit wouldbegin to increase and the electric amperage holding back the stringwould then begin to decrease. To maintain the amperage set by theoperator the rate of descent would slow, thereby returning the amperageto the set value and consequently maintaining the weight on bit at aconstant value. Conversely, if the bit encounters a softer formation ora void, the applied weight on bit would decrease. The amperage wouldthen decrease and the system would attempt to regain the amperagedifferential, allowing the drill string to descend faster.

It should be understood that the control circuit according to thepresent invention may control the coiled tubing injector so that it isoperable at times in the “in hole” mode which results in greaterpenetration drilling. At other times, and in many applications during amajority of the time, the control circuit regulates the coiled tubinginjector so that the injector exerts a controlled upward force on thecoiled tubing while the drilling operation is occurring, with its upwardforce being less than the coiled tubing string weight. This results insubstantially constant weight-on-bit. For each of the embodimentsdisclosed herein, there may be a readout of the weight on bit availableto the operator via a load cell in the injector frame. The operator thusadjusts the power to the injector to achieve the desired weight on bit.Once achieved and placed into a weight on bit mode, the injector systemwill strive to maintain that weight on bit independent of the operator.Those skilled in the art will appreciate that the coiled tubing stringweight is the entire weight of the coiled tubing string and the bottomhole assembly hanging from the rig floor, including drill motors andother downhole equipment, as well as drill dollars added to obtaindesired weight to the bottom hole assembly.

Although specific embodiments of the invention have been describedherein in some detail, this has been done solely for the purposes ofexplaining the various aspects of the invention, and is not intended tolimit the scope of the invention as defined in the claims which follow.Those skilled in the art will understand that the embodiment shown anddescribed is exemplary, and various other substitutions, alterations andmodifications, including but not limited to those design alternativesspecifically discussed herein, may be made in the practice of theinvention without departing from its scope.

1. A control circuit for a coiled tubing injector for raising orlowering coiled tubing into a well, the injector including an injectormotor to power an injector drive mechanism, the injector drive mechanismengaging the coiled tubing to raise or lower the coiled tubing, thecircuit comprising: a circuit controller for varying power to theinjector motor to maintain a substantially constant weight on bit; andthe circuit controller controlling the injector motor to exert acontrolled upward force on the coiled tubing less the coiled tubingstring weight, thereby resulting in a substantially constant weight onbit, said circuit controller comprising a hydraulic circuit controller,which reacts to a hydraulic pressure to maintain a substantiallyconstant weight on bit; a counterbalance valve opening in response tothe circuit controller to power the injector motor for rate ofpenetration movement of the coiled tubing and closing in response to thecircuit controller to prevent the tubing string from descending into thewellbore; a bypass line around the counterbalance valve; and a controlvalve for opening to bypass the counterbalance valve.
 2. A controlcircuit as defined in claim 1, wherein the injector motor is ahydraulically powered motor, and the circuit further comprises: a pumpfor powering the hydraulic motor.
 3. A control circuit as defined inclaim 2, wherein the pump is a variable displacement, variable pressurehydraulic pump.
 4. A control circuit as defined in claim 1, wherein thecontrol valve is an electrically operated solenoid valve which opens toopen the bypass line around the counterbalance valve.
 5. A controlcircuit as defined in claim 1, further comprising: a pressure controlvalve adjustable to vary fluid pressure to the motor to hold the coiledtubing string substantially stationary.
 6. A control circuit as definedin claim 5, further comprising: a pressure relief valve for controllablerelieving fluid pressure to the motor; and a bleed line to replenishbled hydraulic fluid to a hydraulic circuit.
 7. A control circuit for acoiled tubing injector for raising or lowering coiled tubing into awell, the injector including a pair of hydraulically powered injectormotors to power an injector drive mechanism, the injector drivemechanism engaging the coiled tubing to raise or lower the coiledtubing, the circuit comprising: a circuit controller for varying powerto the injector motors to maintain a substantially constant weight onbit responsively to changes in a hydraulic pressure; a pump for poweringthe injector motors; and the circuit controller controlling the injectormotors to exert an upward force on the coiled tubing less drill stringweight, thereby resulting in a substantially constant weight on bit; acounterbalance valve opening in response to the circuit controller topower the injector motors for rate of penetration movement of the coiledtubing and closing in response to the circuit controller to prevent thetubing string from descending into the wellbore; a bypass line aroundthe counterbalance valve; and a control valve for opening to bypass thecounterbalance valve.
 8. A control circuit as defined in claim 7,wherein the pump is a variable displacement, variable pressure hydraulicpump.
 9. A control circuit as defined in claim 7, wherein the controlvalve is an electrically operated solenoid valve which opens to open thebypass line around the counterbalance valve.
 10. A control circuit for acoiled tubing injector for raising or lowering coiled tubing into awell, the injector including a pair of hydraulically powered injectormotors to power an injector drive mechanism, the injector drivemechanism engaging the coiled tubing to raise or lower the coiledtubing, the circuit comprising: a circuit controller for varying powerto the injector motors to maintain a substantially constant weight onbit responsively to changes in a hydraulic pressure; a pump for poweringthe injector motors; and the circuit controller controlling the injectormotors to exert an upward force on the coiled tubing less drill stringweight, thereby resulting in a substantially constant weight on bit; anda pressure control valve adjustable to vary fluid pressure to the motorto hold the coiled tubing string substantially stationary.
 11. A controlcircuit as defined in claim 10, further comprising: a pressure reliefvalve for controllable relieving fluid pressure to the motor; and ableed line to replenish bled hydraulic fluid to the hydraulic circuit.12. A control circuit for a coiled tubing injector for raising orlowering coiled tubing into a well, the injector including a pair ofhydraulically powered injector motors to power an injector drivemechanism, the injector drive mechanism engaging the coiled tubing toraise or lower the coiled tubing, the circuit comprising: a circuitcontroller for varying power to the injector motors to maintain asubstantially constant weight on bit responsively to changes in ahydraulic pressure; a pump for powering the injector motors; and thecircuit controller controlling the injector motors to exert an upwardforce on the coiled tubing less drill string weight, thereby resultingin a substantially constant weight on bit; and a pressure control valveto control pressure to the injector motors such that the motor speedvaries to maintain a substantially constant weight bit.
 13. A controlcircuit for a coiled tubing injector for raising or lowering coiledtubing into a well, the injector including an injector motor to power aninjector drive mechanism, the injector drive mechanism engaging thecoiled tubing to raise or lower the coiled tubing, the circuitcomprising: a circuit controller for varying power to the injector motorto maintain a substantially constant weight on bit; and the circuitcontroller controlling the injector motor to exert a controlled upwardforce on the coiled tubing less the coiled tubing string weight, therebyresulting in a substantially constant weight on bit, said circuitcontroller comprising a hydraulic circuit controller, which reacts to ahydraulic pressure to maintain a substantially constant weight on bit; acounterbalance valve opening in response to the circuit controller topower the injector motor for rate of penetration movement of the coiledtubing and closing in response to the circuit controller to prevent thetubing string from descending into the wellbore; a bypass line aroundthe counterbalance valve; a control valve for opening to bypass thecounterbalance valve; and a pressure control valve to control pressureto the injector motor such that the motor speed varies to maintain asubstantially constant weight bit.